Study On Absolute Measurement Of Air Kerma For Cs-137γ-ray

With the rapid development of nuclear science and its widely use, the need of radiation protection and the measurement technology of radiation dose in nuclear unit such as large nuclear power plants、nuclear medical equipment and nuclear research department has become more and more urgent. Therefore the gamma-ray absolute measurement and protection in the medical, scientific research, production plays an increasingly important role in the life. Every year the number of instruments used in gamma-ray alarm protection continue to rise, but many technical indicators involved cannot be traced back to the protection class gamma benchmark, this situation need to be solved. The air kerma as a important physical quantities in ionizing radiation metrology field, is one of the important research foundation of metrological work. The Cs-137source used as a protective energy source infield of radiation protection, y-ray air kerma absolute measurement has become the important basis of radiation protection research.This paper relied on the research subject of National Institute of Metrology: Cs-137air kerma benchmark established (25-AJDYX1111) and Cs-137benchmark positioning system (25-JB1266). The subject is based on the ionizing radiation metrology theory, through experiment and research, to build the air kerma primary standard for y-ray on protective level in our country. The establishment of the benchmark device can solve the problem of radiation protection class of instrument quantity traceability in domestic. Cs-137y-ray air kerma has been listed as international key comparison values. After the task completed, it will fill in the blank of the air kerma primary standard for Cs-137y-ray on protective level in our country.This paper mainly studies the Cs-137y-ray air kerma absolute measurement. This mainly includes the graphite cavity ionization chamber’s design, production, production, assembly, testing, the structure method and improvements of the weak current measurement system, the corrections of measured values for cavity theory study. Main hardware devices are:a set of home-made graphite cavity ionization chamber、the apparatus used in the weak current measurement system and other auxiliary devices used in the correction experiment. These match up with the data acquisition software of the weak measurement system, completed a series of absolute measurement experiment and correction factor measurement experiment.Due to the development of cavity theory, the cavity ionization chamber has become commonly used in radiation measurement in the field of radiation dosimetry. Graphite cavity ionization chamber was used for Cs-137y-ray absolute measurement. At the beginning of the set of ionization chamber design, the main consideration to achieve electronic balance condition, the selection of material and the influence of the collector to the internal electric field, finally decided to use nominal10cm3cylindrical graphite cavity ionization chamber used as absolute measurement cavity ionization chamber.In this paper, accurate measurement of the spherical and cylindrical chamber’s volume is studied. When I measure the volume of spherical chamber, I put the water into the chamber, through calculating the volume of water in the cavity to estimate the volume of the chamber. The volume of cylindrical chamber adopts the block measurement method. I measured the inside high and outside, top plate and the thickness in the base of the cylinder separately, finally through calculating the height of internal cylindrical chamber and using inside diameter micrometer to measure the inner diameter of the graphite cylindrical, the volume of the cavity is calculated. The way to measure the volume of the inside the collector is the combination of measurement and calculation method, collect the tip of the pole as a result of the limitation of the machining accuracy, I cannot directly measure it according to half spherical volume, thus to make two different shape (cylinder head and trapezoidal head) simulation approximate calculation, and the final volume is given. The whole volume of cylindrical volume removed the volume of the collection, the remaining is the effective volume of cavity ionization chamber.Absolute measurement use two method:Townsend null compensation and high gain capacitor negative feedback. The design uses almost the same hardware system, through a simple connector plug to realize the transformation of the two methods, which can verify the differences of the two methods and select the optimal scheme.Another focus of this paper is method of a series of correction factor determine to the cavity theory. It includes the correction of wall attenuation, scattering and mean origin of electrons,kwall; the correction of recombination losses for ionization chamber, ks; the stem scattering correction of contribution to ionization chamber,kst; the axial and radial non uniformity correction of radiation field at reference point, kan, km; the correction of humidity to the measured current, kh. Part of the correction factor adopted both physics experiment measurement method and Monte Carlo calculation method to determine. With the development of Monte Carlo simulation software, part of the correction factor is more suitable for the numerical simulation method. This article adopts the Monte Carlo simulation software EGSnrc calculate part of the correction factor, this software is also commonly used in the dose simulation in the world. First using software to make subject material cross section database, and then according to the parameters of each part in graphite cavity ionization chamber, make its geometry model. Finally according to the type of simulation calculation to set the corresponding parameters and run a certain number of particles to calculated the correction term. Part of the correction factor is determined by both two kinds of methods, and the differences of the two methods can be analyzed. For the physical parameters which involved in absolute measurement, it can be decided by Monte Carlo simulation method in addition to the reference international experimental.The paper finally discussed the uncertainty analysis of each part.